The present invention relates to multi-layer resin films having excellent oxygen gas barrier properties (hereinafter abbreviated in some cases as xe2x80x9coxygen gas barrier filmsxe2x80x9d) and a process for their production. More particularly, it relates to multi-layer resin films having excellent oxygen gas barrier properties even at high humidity and a process for their production.
The multi-layer resin films of the present invention have excellent oxygen gas barrier properties, particularly even at high humidity, and therefore, they can be applied in various fields. The oxygen gas barrier films of the present invention can be used, mainly in the field of packaging, for example, suitably for food packaging films.
Various resin films have been used as packaging materials to pack or package various articles including food and drink, medicines, chemicals, and miscellaneous goods for daily use. For example, polypropylene films have been widely used for many applications because of their excellent characteristics such as processability, transparency, and heat resistance. However, packaging materials for articles such as food and medicines, of which quality may be deteriorated by oxygen gas, have been required to have high oxygen gas barrier properties (i.e., oxygen gas blocking properties) for preserving or keeping the quality of these articles to be packaged therewith. Therefore, polypropylene films having no sufficient oxygen gas barrier properties have met difficulty in their application.
As the resin films having improved oxygen gas barrier properties in comparison with resin films having low oxygen gas barrier properties, such as polypropylene films, there have been widely used for many applications, for example, polyvinylidene chloride resin (hereinafter abbreviated in some cases as xe2x80x9cPVDC resinxe2x80x9d) coated films prepared by coating PVDC resins on base resin films such as polypropylene films. PVDC resin coated films have good oxygen gas barrier properties even at high humidity because of their extremely low moisture absorption. However, they have a serious problem that evolution of chlorine gas will be caused in their incineration. In recent years, there has been a particular demand for resin films having no chlorine content as part of anti-dioxin measures and further having high oxygen gas barrier properties. As such resin films, there have been proposed, for example, polyvinyl alcohol resin (hereinafter abbreviated in some cases as xe2x80x9cPVA resinxe2x80x9d) coated films prepared by coating PVA resins on base resin films. PVA resin coated films have excellent oxygen gas barrier properties at low humidity. However, they have deteriorated oxygen gas barrier properties with a rise in humidity because of their high moisture absorption, and therefore, they have no sufficient practical use as the substitutes for PVDC resin coated films.
As the resin films having improved moisture absorption in comparison with such PVA resin coated films, there have been proposed, for example, ethylene vinyl alcohol copolymer resin (hereinafter abbreviated in some cases as xe2x80x9cEVOH resinxe2x80x9d) coated films prepared by coating EVOH resins on base resin films. EVOH resin coated films have improved moisture absorption. However, they have lower oxygen gas barrier properties at low humidity in comparison with PVA resin coated films, and therefore, they have no sufficient oxygen gas barrier properties.
JP-A 3-30944 discloses a method for adding colloidal hydrated phyllosilicate compounds having swell properties to PVA resin coating solutionsxe2x80x9d, and JP Patent No. 2,789,705 discloses polyvinyl alcohols modified with at least one of colloidal hydrated phyllosilicate compounds having swell properties or compounds containing silyl groups in the molecules. However, any coated films prepared by coating these materials on base resin films need high cost of production, and therefore, they are not sufficient as the substitutes for PVDC resin coated films with low cost of production.
Further, JP-A 49-64676 discloses a technique of forming PVA resin layers on polyethylene terephthalate films as the base resin films and then stretching the laminated films, and this reference teaches that resin films prepared by applying aqueous polyvinyl alcohol solutions onto uniaxially-stretched polypropylene resin films as the base resin films and then stretching the laminated films have insufficient adhesion properties between the PVA resin layers and the base resin films.
In any case, there have not yet been provided multi-layer resin films having high oxygen gas barrier properties, which can take the place of PVDC resin coated films, particularly multi-layer resin films which can exhibit excellent oxygen gas barrier properties even at high humidity and which can be produced at low cost.
Under these circumstances, the present inventors have extensively studied to obtain resin films having excellent oxygen gas barrier properties, particularly even at high humidity, and further having no chloride content, as well as a process for producing such oxygen gas barrier films at low cost. As a result, they have found that excellent oxygen gas barrier properties even at high humidity can be exhibited by multi-layer resin films comprising polyvinyl alcohol resin layers (hereinafter abbreviated in some cases as xe2x80x9cPVA layer(s)xe2x80x9d) having specific values of crystallinity parameter, which have been formed, together with interposed adhesive layers, on at least one surface of biaxially-stretched polypropylene resin films, thereby completing the present invention.
Thus the present invention provides a multi-layer resin film having excellent oxygen gas barrier properties, comprising a biaxially-stretched polypropylene resin film, an adhesive layer formed on at least one surface of the polypropylene resin film, and a polyvinyl alcohol resin layer formed on the adhesive layer, wherein the polyvinyl alcohol resin layer has a value of crystallinity parameter (CP (M//) xe2x8axa5) not less than 0.8.
The multi-layer resin film having excellent oxygen gas barrier properties of the present invention can be prepared by forming an adhesive layer on at least one surface of an unstretched polypropylene resin film; stretching the film in one direction; forming a polyvinyl alcohol resin layer on the adhesive layer; and stretching the film in another direction perpendicular to the direction of previous stretching.